The catalytic water gas shift conversion process is well known and is commonly used in processes which manufacture hydrogen gas. In the water gas shift reactor, carbon monoxide is combined with water to yield carbon dioxide and hydrogen according to the following formula: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2.
This reaction, commonly known as the water gas shift reaction, is highly exothermic, liberating about 16,700 BTUs for each pound mole of carbon monoxide converted. Water gas shift reactors are often used to reduce the amount of carbon monoxide present in a gas stream typically composed of water vapor, methane, carbon monoxide, carbon dioxide and hydrogen.
Water gas shift reactors are particularly useful in hydrocarbon fueled electric power generation systems which utilize ion exchange membrane based electrochemical fuel cells. In these systems, natural gas, or some other suitable hydrocarbon gas, is first reformed in a catalytic hydrocarbon reformer to yield a mixture of hydrogen, carbon dioxide and small amounts of carbon monoxide. This gas mixture is commonly referred to as reformate. The reformate gas is then introduced into the water gas shift reactor, where the carbon monoxide concentration is reduced in order to avoid poisoning by the carbon monoxide of the catalyst employed in the fuel cells and to produce additional hydrogen fuel. In many instances, the reformate stream exiting the water gas shift reactor is introduced into a selective oxidizer, which further reduces the level of carbon monoxide present in the stream.
The conventional water gas shift reactor is an adiabatic bed in which the process gas temperature increases as the amount of carbon monoxide is reduced by the water gas shift reaction. Practically speaking, due to equilibrium limitations, catalyst activity, and catalyst thermal limits, conventional water gas shift reactors are generally incapable of reducing the carbon monoxide concentration of a reformate stream much below 0.8%.
The carbon monoxide concentration of a reformate stream can be reduced below 0.8% through the use of a second adiabatic catalyst bed. However, because of equilibrium limitations, a second adiabatic bed would necessarily operate at a lower temperature relative to the first bed. As a result, the catalyst activity in the second adiabatic bed would be so low that the size of the catalyst bed necessary to achieve any meaningful reduction in carbon monoxide concentration would be prohibitively large from an economic and efficiency standpoint.
Therefore, it is an object of the present invention to provide a water gas shift conversion process, and apparatus therefor, that is capable of economically reducing the carbon monoxide concentration of a reformate stream well below the concentration achieved in conventional processes.
It is a further object of the present invention to provide a water gas shift conversion process which reduces the concentration of carbon monoxide in a reformate stream in order to avoid poisoning of the catalyst employed in the associated fuel cell or cells to which the reformate stream is introduced.
It is a still further object of the present invention to provide a water gas shift conversion process which utilizes one adiabatic reactor bed and one cooled reactor bed.